A typical cellular wireless network includes a number of base stations each radiating to define a respective coverage area in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In particular, each coverage area may operate on one or more carriers each defining a respective frequency bandwidth of coverage. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Further, a cellular wireless network may operate in accordance with a particular air interface protocol (radio access technology), with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Long Term Evolution (LTE) (using Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WIFI), BLUETOOTH, among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and other functions related to air interface communication.
In accordance with a recent version of the LTE standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, with each carrier being divided primarily into subcarriers spaced apart from each other by 15 kHz. Further, the air interface is divided over time into a continuum of 10-millisecond frames, with each frame being further divided into ten 1-millisecond subframes or transmission time intervals (TTIs) that are in turn each divided into two 0.5-millisecond segments. In each 0.5 millisecond segment or in each 1 millisecond TTI, the air interface is then considered to define a number of 12-subcarrier wide “resource blocks” spanning the frequency bandwidth (i.e., as many as would fit in the given frequency bandwidth). In addition, each resource block is divided over time into symbol segments of 67 μs each, with each symbol segment spanning the 12-subcarriers of the resource block and thus supporting transmission of symbols in “resource elements.”
The LTE air interface then defines various channels made up of certain ones of these resource blocks and resource elements. For instance, on the downlink, certain resource elements across the bandwidth are reserved to define a physical downlink control channel (PDCCH) for carrying control signaling from the base station to UEs, and other resource elements are reserved to define a physical downlink shared channel (PDSCH) for carrying bearer data transmissions from the base station to UEs. Likewise, on the uplink, certain resource elements across the bandwidth are reserved to define a physical uplink control channel (PUCCH) for carrying control signaling from UEs to the base station, and other resource elements are reserved to define a physical uplink shared channel (PUSCH) for carrying bearer data transmissions from UEs to the base station.
In a system arranged as described above, when a UE enters into coverage of a base station, the UE may engage in attach signaling with the base station, by which the UE would register to be served by the base station on a particular carrier. Through the attach process and/or subsequently, the base station and supporting LTE network infrastructure may establish for the UE one or more bearers, essentially defining logical tunnels for carrying bearer data between the UE and a transport network such as the Internet.
Once attached with the base station, a UE may then operate in a “connected” mode in which the base station may schedule data communication to and from the UE on the UE's established bearer(s). In particular, when a UE has data to transmit to the base station, the UE may transmit a scheduling request to the base station, and the base station may responsively allocate one or more upcoming resource blocks on the PUSCH to carry that bearer traffic and transmit on the PDCCH to the UE a downlink control information (DCI) message that directs the UE to transmit the bearer traffic in the allocated resource blocks, and the UE may then do so. Likewise, when the base station has bearer traffic to transmit to the UE, the base station may allocate PDSCH resource blocks to carry that bearer traffic and may transmit on the PDCCH to the UE a DCI message that directs the UE to receive the bearer traffic in the allocated resource blocks, and the base station may thus transmit the bearer traffic in the allocated resource blocks to the UE. LTE also supports uplink control signaling on the PUCCH using uplink control information (UCI) messages. UCI messages can carry scheduling requests from UEs, requesting the base station to allocate PUSCH resource blocks for uplink bearer data communication.
Moreover, a revision of LTE known as LTE-Advanced now permits a base station to serve a UE with “carrier aggregation,” by which a base station schedules bearer communication with the UE on multiple carriers at a time. With carrier aggregation, multiple carriers from either contiguous frequency bands or non-contiguous frequency bands can be aggregated to increase the bandwidth available to the UE. Currently, the maximum bandwidth for a data transaction between a base station and a UE using a single carrier is 20 MHz. Using carrier aggregation, a base station may increase the maximum bandwidth to up to 100 MHz by aggregating up to five carriers. Each aggregated carrier is referred to as a “component carrier.”
Although serving a UE with carrier aggregation can help improve throughput for the UE, other technologies could also help improve throughput for the UE. One such technology is known as multiple-input multiple-output (MIMO) with spatial multiplexing.
In particular, MIMO provides for air interface communication concurrently on multiple different radio-frequency propagation paths, from multiple transmit-antennas at the transmitting end (e.g., at the base station or the UE) to multiple receive-antennas at the receiving end (e.g., at the UE or the base station). With spatial multiplexing, when the transmitting end has data to transmit to the receiving end on a given carrier, the data is multiplexed onto multiple antenna output ports and thus onto multiple RF propagation paths, so that a separate portion of the data is transmitted respectively on each propagation path. In practice, each propagation path is considered to be a MIMO “layer” assigned to the given carrier.
Moreover, MIMO communication service on a carrier could be characterized by how many transmit antennas (or transmit antenna groups), T, are used and how many receive antennas (or receive antenna groups), R, are used, as T×R MIMO service. Further, if T and R are equal, then the number of MIMO layers assigned to the carrier could be considered equal to T and R, whereas if T and R are different, then the number of MIMO layers assigned to the carrier could be considered the lesser of the two. Thus, for example, MIMO service on the carrier with two transmit antennas and two receive antennas (2×2 MIMO) could be considered to have two layers, MIMO service on the carrier with four transmit antennas and four receive antennas (4×4 MIMO) could be considered to have four layers, and MIMO service on the carrier with two transmit antennas and one receive antenna (2×1 MIMO) or with just one transmit antenna and one receive antenna (1×1 MIMO—still MIMO, but effectively single-input single-output (SISO)) could be considered to have just one layer. Other examples are possible as well.
When a base station serves a UE on a single carrier using MIMO with N layers, the base station could specify the MIMO configuration in the base station's DCI message to the UE when scheduling data communication to or from the UE to occur on particular air interface resources. In accordance with that MIMO specification, the base station and UE may then each make use of the indicated number of antennas for their air interface communication with each other, with the data communication being spatially multiplexed over the indicated number of layers. Thus, for communication in a given subframe, the base station could allocate particular air interface resources for communication to or from the UE and could specify use of N MIMO layers on the carrier; and all N of the MIMO layers could then share those allocated air interface resources, being distinguished from each other by at least their spatially separate RF propagation paths.
Likewise, when a base station serves a UE on a multiple carriers concurrently and uses MIMO with N layers, the base station could specify the MIMO configuration in its DCI message to the UE when scheduling data communication to or from the UE to occur on particular air interface resources respectively on each carrier. In accordance with that MIMO specification, the base station and UE may then each make use of the indicated number of antennas for their air interface communication with each other, with the data communication being spatially multiplexed over the respectively indicated number of layers on each carrier. Thus, for communication in a given subframe, the base station could allocate particular air interface resources respectively on each of the carriers. And on each respective carrier, the respectively assigned layers could share the air interface resources allocated on the carrier, being distinguished from each other by at least their spatially separate RF propagation paths.